1. Field of the Invention
The present invention relates to a latent curing agent for epoxy curing resins. More particularly, it relates to a latent curing agent for epoxy resins which causes rapid resin curing at moderate, elevated temperatures and which gives epoxy resin compositions having excellent storage stability at room temperature.
2. Description of the Prior Art
One-pack type epoxy resins are preferable to the conventional two-pack type epoxy resins because the former cannot be misformulated and can be used continuously. One-pack type epoxy resins require a so-called latent curing agent which does not react with epoxy compounds at room temperature, but upon heating, reacts with epoxy compounds to effect curing.
Heretofore, several latent curing agents have been proposed. Representative examples include boron trifluoride-amine adduct, dicyandiamide, and dibasic acid dihydrazide. Boron trifluoride-amine adduct is hard to treat because it is very hygroscopic and it adversely affects the physical properties of the cured resin. The latter two curing agents are useful in formulating epoxy resin compositions having excellent storage stability, but full curing by means of these compounds can only be achieved by heating the resin at temperatures higher than 150.degree. C. for a long time. To save energy and not to damage surrounding components, rapid curing at a low temperature is required.
It is known that an amine type curing agent contributes improved workability to the system being converted into an adduct with an epoxy resin. It has recently been disclosed that an adduct obtained by the reaction of an imidazole compound with an epoxy compound is a comparatively good latent curing agent. (See U.S. Pat. No. 4,066,625.) For example, an adduct obtained by the reaction of 2-ethyl-4-methylimidazole with Epon 834 (bisphenol A type epoxy resin; a product of Shell Chemical Co., epoxy equivalent 230.about.270) has a softening temperature of 100.degree. to 125.degree. C. However, it is a general observation that formulated resins which have good reactivity have poor storage stability, or if the resin has good storage stability, it has poor reactivity. Moreover, the range of good reaction ratios is very narrow. Epoxy resin compositions which incorporate an adduct therein and which are obtained by the reaction of one equivalent of epoxy groups in the epoxy resin (Epon 834) with one equivalent of the secondary amino group of 2-ethyl-4-methylimidazole with curing at 100.degree. C. for thirty minutes, exhibit poor storage stability (4 days at 35.degree. C., 2 days at 40.degree. C.). Epoxy resin compositions which incorporate an adduct therein and which are obtained by the reaction of two equivalents of epoxy groups in the epoxy resin (Epon 834) with one equivalent of 2-ethyl-4-methylimidazole, have good storage stability (&gt;14 days at 35.degree. C.), but exhibit poor reactivity. These adducts cure at low temperatures when they are used with other curing agents, and accordingly are superior as curing agents to an adduct of an aliphatic amine with an epoxy compound as latent curing agents. However, they are insufficient as latent curing agents which cure at 100.degree..about.130.degree. C. and exhibit superior storage stability.
It is known that an adduct obtained by the reaction of N-methylpiperazine with an epoxy compound can be used as a latent curing agent (see U.S. Pat. No. 4,268,656). However, this adduct alone is not effective as a latent curing agent and must be used with another curing agent.
In short, the adduct of an aliphatic amine and an epoxy compound does not improve the storage stability of epoxy resin very much. In the case of a specific amine compound, i.e., the adduct which is formed between the amine compound and the epoxy compound, it is rare to find a curing agent which satisfies both reactivity and storage stability requirements for a one-pack type epoxy resin. A need therefore continues to exist for an improved curing agent for a one-pack type epoxy resin.